In recent years, the demands for the miniaturization of electronic instruments, higher frequency processing speeds, weight savings, and reduction in manufacturing costs are driving down the solder bump density and requiring an increase on the number of solder bumps per package. The industry has progressed from the use of through hole pin technology in which the pin leads on the semiconductor package are physically inserted through corresponding holes in the printed circuit board (PCB), to surface mount technology (SMT) to improve both manufacturing costs and density.
There exist numerous techniques for mounting semiconductor packages to PCB's using SMT. Currently, the popular technique is to mount leadless surface devices, and in particular, the ball grid array (BGA) type semiconductor packages in which solder ball leads on the substrate of an encapsulated semiconductor package contact with the landing pads on a PCB. The BGA is distinguished by the presence of solder bumps on the underside of the encapsulated semiconductor package. BGA's offer the advantage of providing higher chip density, reliability, and efficiency. With continual improvements in the miniaturization of BGA's, the size of the die has approached that of the encapsulant in a flip chip BGA. These particular BGA packages are commonly referred to as chip scale packages (CSP).
BGAs are generally manufactured by mounting a bare integrated circuit (IC), also known as a die or silicon die, on a double sided connecting substrate for supporting the IC, and establishing a connection between the leads on the IC and contacts on the connecting substrate. In one configuration, the connection is established by connecting wires running from terminals on the periphery of the IC, or from terminals on the top surface of the IC, to the external terminals on a wired substrate. The IC is generally adhesively adhered to the connecting substrate using a thermoset or thermoplastic adhesive. FIG. 1 illustrates such configuration, known as a transfer molding chip or a BGA encapsulated package. The IC 1 is adhesively mounted with a staking adhesive 2 to the connecting substrate 3 having solder bumps 6 on its bottom side. The IC is connected by wires 4 welded from the IC 1 to contacts on the connecting substrate 3. The wiring, IC, and staking adhesive are encapsulated with an encapsulating resin 5, typically a thermosetting epoxy resin. The staking adhesive is also typically a thermosetting filled epoxy resin which is heat cured.
In a flip chip design, however, solder bumps instead of wires are formed on one side of the IC, the IC is flipped over and laid up onto the corresponding solder bumps or other contacts on the connecting substrate, followed by solder reflowing the solder bumps on the IC and the contacts on the connecting substrate to make an electrical connection. In the flip chip design, the gap between the IC and connecting substrate is often underfilled with an epoxy resin. The structure is illustrated in FIG. 2. Underfill is generally accomplished by mounting the IC 1 facedown so that solder balls 7 are in alignment with the contacts on the connecting substrate 2 having solder balls 6 along its bottom surface, placing the assembly into a mold with an underfill resin, and forcing the underfill resin 3 to flow into the gap 4 between the IC and the connecting substrate 5 and around the periphery of the of the IC to provide a good seal. The underfill resin is also generally a thermosetting epoxy resin. The structure is then encapsulated 5 with an epoxy resin or other resin encapsulant to form a semiconductor package known as a BGA.
There exist a wide variety of BGA configurations. Common BGA packages include flip chip BGA, ceramic (CBGA), plastic (PBGA), and micro-BGA (MBGA), also known as chip size packages (CSP). A CSP is a type of BGA in which the size of the IC approaches that of the connecting substrate. Each structure has its own advantages and attributes.
BGAs are usually attached to rigid planar PCB's made of FR-4 glass/epoxy composites. In some applications, the BGA's may be attached to flexible PCBs made of polyimide, polyester, or other like materials. BGA packages are mounted onto a PCB by reflow solder processing techniques. BGAs have a grid or array of solder bumps acting as the terminals arranged on the bottom side of the chip package to establish the electrical connection with the landing pads on the PCB. A solder containing lead in the form of a paste or other form in a silk screen is set between the solder bumps on the BGA and the etched terminals or pads on the PCB. The BGA is laid up cold on the solder paste and subsequently aligned with the landing pad on the PCB. The assembly is then passed through an oven and heated, usually by convection, within a time-temperature profile to enable the reflow of the solder. Typical peak solder reflow temperatures range from 200° C. to 260° C. In another method, chip packages may also be surface mounted by placing a small amount of solder flux on the solder bumps themselves. The array is precisely positioned on the PCB landing pads, and the assembly is then heated under an appropriate time-temperature profile to establish the connection.
Many applications containing printed circuit boards require the ability of the printed circuit board to flex, resist vibration, and/or resist impacts. For example, portable electronic devices such as phones, pagers, lap top computers, calculators, digital audio players, cameras, and gaming devices contain PCB's on which are mounted chip packages. These hand held devices are subject to impact by drops or vibration, leading to the failure of the chip package/PCB solder joint. These impacts break the solder joints between the connecting substrate and the landing pads on a printed circuit board. In some cases, a significant number of the solder joints in the solder array of the BGA may separate from the PCB, or the entire BGA may pop off the PCB if the force of the impact or vibration is sufficient.
Adhesion of the chip package to the PCB is becoming more problematic as the technology continues to miniaturize the chip packages, the interconnect density increases, the solder ball density decreases thereby reducing the size of each solder ball and joint, and the gap between the chip package and the PCB continues to decrease. In recent years the size (height and width) of the solder bumps and the spacing between the bumps in the array of the BGA has become much smaller. While there is variance among the types of chip packages, generally bump heights may range from less than one to several millimeters. Commonly used BGA's have centerline pitches (pitches) ranging from 0.5 mm×0.5 mm (or 19.7 mil) in MBGA's and CSP, to 1.50 mm×1.50 mm (or 59 mil) in PBGA and CBGA. The centerline pitch is the distance between the bumps in an array.
Since the contact area between the solder bumps and the landing pads on the PCB has become much smaller, the gap between the chip package and the PCB has also become much smaller. Moreover, the interconnect density is rapidly increasing. The interconnect density in the 1980's was generally on a 100 mil pitch with center-to center spacing of 0.100″ (about 2.5 mm) [1 mm=39.4 mils], yielding an interconnect density of 100 contacts per square inch. In the 1990's, the physical dimensions of the contacts have become smaller and the distance between contacts reduced to 10 mils and often less. Thus the interconnect density has increased to 10,000 or more per square inch.
Several solutions to address the issue of solder joint failure have been proposed. One proposed solution was to add more solder balls to assist with the adhesion of the chip package to the PCB. However, the vibration and drop resistance is not sufficient and the electrically connected solder joints fail. The addition of metal grids and springs to act as shock absorbers has been proposed, but this mechanical solution requires additional space by increasing gap distance and chip size. Another proposal which was placed into commercial practice was to underfill the gap between the connecting substrate and the printed circuit board.
FIG. 3 illustrates a typical configuration of a flip chip BGA mounted on a printed circuit board and underfilled with a resin. The solder bumps 8 on a BGA 1, comprised of an IC 2 connected to the contacts on a connecting substrate 3 through solder bumps 4 and underfilled with a thermosetting resin 5 wherein the IC 2 is encapsulated with an thermosetting resin 6, are mounted on and aligned with the landing pads 10 on a printed circuit board 7, the assembly is solder reflowed, and the gap between the connecting substrate and the printed circuit board is underfilled with a thermosetting epoxy resin 9 to provide adhesion between the board 7 and the connecting substrate 3. When a gap is underfilled, the resin flows across the entire bottom surface of the connecting substrate and between the solder joints.
However, the use of underfills has not provided a complete and economical solution to joint failure in a market which demands ever increasing performance at lower costs. The smaller gap widths and the increased interconnect density have begun to exceed the limitations of underfill technology as a practical means for securing the surface mounted electronic device to the printed circuit board. Using capillary action or forced injection of underfill resins into such small gaps (which today can be as small as 7 mils) containing such high interconnect densities without damaging any of the solder joints while simultaneously providing resistance to the impacts of drops has stretched the limits of underfill technology. While the use of underfill technology has aided in the reduction of solder joint failures in drops for certain applications, the gap height reduction, increase in interconnect density, and decrease in solder ball density have necessitated a search for a new solution which improves the adhesion in new surface mount technologies.
In addition to the problem of adhesion in new surface mounted technologies, the use of underfills introduced a costly and complex manufacturing step due to the specific material design need to achieve performance demands and the manufacturing cycle time. While a wide variety of underfill techniques are used commercially, and other techniques have been proposed, a common manufacturing technique for underfilling the gap between the chip package/PCB gap comprises a first step for heating the resin and heating the board, a second step for dispensing the resin as by injecting the resin or allowing the resin to flow by capillary action under the chip package and into the gap and around the chip package to provide a seal, followed by curing the resin for a period of about 15 minutes to 45 minutes. The underfill technology also requires dispensing the resin uniformly throughout both surfaces of the gap, and throughout the whole of the surfaces, thus requiring the use of large amounts of material in the process. Specialized equipment is needed to reliably and accurately place assembly and apply the required amount of resin. The underfill must be designed to have a viscosity which enables it to wet all surfaces and flow freely into the gap. Moreover, once hardened, most thermosetting resin bonds are permanent, so rework or repair of faulty chips or solder joints becomes impractical.
Accordingly, it would be desirable to provide a process and material which reduces the surface mounted electronic device/printed circuit board assembly manufacturing cycle time. It would also be desirable to provide a process and material which adheres well to the connecting substrate and the printed circuit board in small gaps without risking damage to solder joints, while providing good impact resistance to the assembly. It would also advantageous to provide an assembly which, if once desires, allows for convenient reworking or repair of faulty chips.